1. Field of the Invention
The present invention relates to a semiconductor integrated circuit. More particularly, the invention relates to a semiconductor integrated circuit capable of controlling part of the power being supplied to its flip-flops.
2. Description of the Related Art
MTCMOS (multi-threshold-voltage complementary metal oxide semiconductor) technology has been known as a way to save power in semiconductor integrated circuits. The circuit blocks to which the MTCMOS technology is applied include functional blocks with low thresholds for operating on low voltages and switches for shutting down leak currents in a standby state.
Illustratively, as shown in FIG. 1, a MTCMOS functional block 910 has logic gates 911 and 912 connected to a virtual ground line VSS1 (903). The virtual ground line VSS1 (903) is connected to an actual ground line VSS (901) with a MTCMOS switch 931 connected interposingly therebetween. In a standby mode, a control signal PG is issued to turn off the MTCMOS switch 931 to stop the supply of power, thus inhibiting leak currents.
If the power to all cells is stopped, the data held by flip-flop cells and latch cells will be lost. If these cells are each implemented as a non-MTCMOS functional block 920, then the data held therein is retained but logic gates 921 and 922 in the non-MTCMOS functional block 920 are powered continuously even while the MTCMOS switch 931 is being disconnected. This arrangement thus defeats the power-saving feature originally targeted by the MTCMOS technology.
In an attempt to bypass the difficulty above, MTCMOS flip-flops and MTCMOS latches have been proposed to make up a minimum of non-MTCMOS elements that keep data held intact in each cell. A similar technique involves having master flip-flops constituted by low-threshold transistors and slave flip-flops by high-threshold transistors that remain active in the standby mode, thereby averting the erasure of stored data (e.g., see Japanese Patent Laid-Open No. Hei 11-284493, with reference to FIG. 4).